1. Field of the Invention
The present invention generally relates to computer networking, and more particularly to a method and system for generating worst-case test patterns of baseline wander.
2. Description of Related Art
Ethernet is a packet-based computer networking technology that is widely used in constructing local area networks. Fast Ethernet or 100BASE-TX, for example, transfers data at a nominal rate of 100 Mbits/sec. In a Fast Ethernet system, as shown in FIG. 1, data are transferred through transformer 1A, transformer 1B and unshielded twisted-pair (UTP) cables 2 between a transmitter 3 and a receiver 4. The transmitter 3, however, functions as a high-pass filter that decays or blocks DC component of the transferred signals, resulting in a baseline wander effect. FIG. 2A and FIG. 2B are exemplary signal waveforms of the transformer 1A at the transmitter side and the transformer 1B at the receiver side, respectively. It is observed that the baseline of the signal waveform at the receiver side suffers by the baseline wander effect. The baseline wander may not be effectively resolved even when a scrambler is used in the transmitter to disperse the power spectrum. Actually, with a scrambler baseline wander can occur whenever transferred packets have correlation with the output of the scrambler.
In order to objectively test the performance of a receiver (e.g., receiver 4) regarding the baseline wander effect, the standard ANSI 263-1995 Annex A.2 defines worst-case test patterns, which are commonly called killer packets. Shown in FIG. 3 is an implementation utilizing killer packets. Specifically, the standard-defined killer packets are pre-stored in a memory device such as a read-only memory (ROM) 30 with a size of at least 2047 nibbles. A killer packet generator 31 waits for the scrambler of a transmitter 32 to reach a specific state (Scram_State) 0x79D as required by the standard. Upon detecting the specific state, the killer packet generator 31 accesses the ROM 30 to obtain the standard-defined killer packets, and then transfers the packets to the transmitter 32.
According to the conventional system for transmitting killer packets as demonstrated above, a memory device such as the ROM 30 is needed for pre-storing the killer packets. This disadvantageously increases cost, power consumption and circuit area. Moreover, the killer packet generator 31 cannot transfer the killer packets until the specific scram state 0x79D has been reached, therefore resulting in excessive latency, which may last up to 82 μs.
For the reason that such a conventional system could not effectively solve the baseline wander effect, a need has arisen to propose a novel scheme for generating worst-case test patterns of baseline wander in an effective and economical manner.